The present invention relates to rotating X-ray tubes and, more particularly, to an interface between dissimilar metals in X-ray tube construction.
The X-ray tube is used in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical X-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into an axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the X-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is sometimes referenced electrically to ground. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
In an X-ray tube device with a rotatable anode, the target consists of a disk made of a refractory metal such as tungsten. The X-rays are generated by making the electron beam collide with this target while the target is being rotated at high speed. Rotation of the target is achieved by driving the rotor provided on a support shaft extending from the target. Such an arrangement is typical of rotating X-ray tubes and has remained relatively unchanged in concept of operation since its introduction. However, the operating conditions for x-ray tubes have changed considerably in the last two decades.
State-of-the-art X-ray tubes utilize large (200 mm diameter, 4.5 kg) cantilever mounted, targets rotating at speeds as high as 10,000 rpm. Extremely large temperature changes occur during the operation of the tube, ranging from room temperature to temperatures as high as 1600.degree. C., produced by the deceleration of fast electrons in the tungsten-rhenium layer of the target track.
For the purposes of heat management and safeguarding of components such as bearings, materials with low thermal conductivity are placed in the heat path. In general, such materials have much higher coefficient of thermal expansion than the other materials used in an X-ray tube. However, these components have to be joined to the others in some fashion (i.e., welding, brazing, bolting, etc.). At these joints, the higher level of growth may cause yielding of the components which grow at a smaller rate.
Balance retention at high rotating speeds and high temperatures is extremely crucial. A typical unbalance specification for larger tubes at the time of shipping is 5 g-cm in either the target or rotor planes. Approximately 5% of manufactured tubes with large targets (165 mm diameter, 2.7 kg) are unusable due to high unbalance. A shift of 19 .mu.m of the target center of gravity will produce this amount of unbalance. As anodes become larger and heavier, the amount of shift that will exceed the unbalance specification becomes less. For the latest target size (diameter of approximately 200 mm and mass of approximately 4.5 kg) a shift of 11 .mu.m will exceed the unbalance specification. These small shifts can easily occur because of the large temperature changes, combined with the use of materials that have different coefficients of thermal expansion. Furthermore, the selection of compatible materials for joints is often limited by the operating temperature, material strength and material expansion properties. Additionally, bolted, brazed, and welded joints are a primary source of unbalance.